Many conventional FeRAM devices include a horizontal ferrocapacitor structure, in which a stack of layers is formed including top and bottom electrodes sandwiching a ferroelectric layer. An alternative “vertical capacitor” structure was suggested in U.S. Pat. No. 6,300,653, the disclosure of which is incorporated herein by reference. A vertical capacitor includes a ferroelectric element sandwiched between electrodes to either side, all at substantially the same level in the FeRAM device.
The vertical capacitors are typically formed over a substructure. The substructure includes various electronic components buried in a matrix (e.g. of TEOS (tetraethylorthosilicate)). The substructure further includes conductive plugs connected to the electronic components, and which extend upwards through the matrix. The upper ends of the plugs typically terminate in TiN/Ir barrier elements, having a top surface flush with the surface of the matrix.
Conventionally, in a process for forming a vertical capacitor, an insulating layer of amorphous Al2O3 is formed over the surface of the matrix, and a thicker layer of ferroelectric material is formed over that, and then crystallized in an oxygen atmosphere. The ferroelectric material may be a perovskite such as PZT (PbZrTiO3). The Al2O3 layer acts as a seed layer for crystallization of the PZT (or other ferroelectric), and has the further function of inhibiting oxygen diffusion into the substructure during the PZT crystallization.
Hardmask elements are then deposited over selected areas of the PZT layer, and the portions of the PZT and Al2O3, which are not protected by the hardmask elements are etched all the way through, forming openings.
The openings are then filled with conductive material such as IrO2, by depositing IrO2 over the entire structure, and chemical-mechanical planarization (CMP) polishing is performed to form a flat upper surface, which is partly the PZT and partly the conductive material. Then, an Al2O3 layer is formed over the surface. The elements of IrO2 constitute electrodes, while the remaining PZT elements constitute the dielectric elements of the ferrocapacitors. At least some of the electrodes may be in electrical contact with the plugs, via the barrier elements.
The vertical capacitor structure has great potential for reducing the cell size, especially if the angle between the horizontal direction and the sides of the remaining PZT elements is high. However, for the PZT to perform effectively it should have a high degree of crystallinity, and furthermore the crystallization should have the correct orientation. Usually, despite the seed layer effect of the Al2O3, PZT deposited on alumina still shows poor crystallinity, with random orientation. This is illustrated in FIG. 1, which shows a diffraction spectrum for various angles for three different types of substrate, corresponding to the lines 1, 2, 3. Line 1 is HT Al2O3 (here “HT” means deposited at a high temperature; line 1 corresponds to 400° C.), line 2 is HT Al2O3 with added oxygen (that is, oxygen is a component of the sputter gas. Thus, whereas line 1 was produced using only Ar as sputter gas, line 2 used an Ar/O2 (ratio 30/20) mixture), line 3 is the case that the Al2O3 substrate is formed by atomic layer deposition (ALD). The peak indicative of the PZT (111) orientation is a position of about 38.5°.
It is known that the crysallinity can be increased by introducing a very thin seed layer (such as TiO2) over the amorphous Al2O3 before the PZT is deposited. FIG. 2 shows the diffraction spectrum for various types of seed. Line 4 shows the case that there is no seed layer (note that line 4 on FIG. 2 is different from line 1 on FIG. 2 due to slightly different deposition conditions), line 5 shows the case that a seed layer is formed of Ti having a thickness of 25 Å, and line 6 shows the case that a seed layer of Ti having a thickness of 25 Å is formed and, before the PZT is deposited, the Ti is oxidized to TiO2. Although the (111) peak at 38.5° is higher in lines 3 and 6 than in lines 1, 2, 4 and 5, it is still relatively low. Also, the PZT morphology is still very poor including voids and nucleation defects. Two electron microscope images, obtained in the experiment illustrated as line 6 of FIG. 2, are shown as FIGS. 3(a) and (b). In FIG. 3(a), the TEOS layer is shown as 7, the Al2O3 as 8 and the PZT as 9. The PZT includes regions indicated by the ovals in FIG. 3(a) with voids and nucleation defects. Furthermore, as shown in FIG. 3(b), the Al2O3 layer 8 and especially the TEOS layer 7 include darker “mushrooms” where lead (Pb) has diffused into the TEOS.